TRANSMISSION MEDIUM

A transmission medium is a material substance (solid, liquid, gas, or plasma) that can propagate energy waves. For example, the transmission medium for sound received by the ears is usually air, but solids and liquids may also act as transmission media for sound.

The absence of a material medium in vacuum may also constitute a transmission medium for electromagnetic waves such as light and radio waves. While material substance is not required for electromagnetic waves to propagate, such waves are usually affected by the transmission media they pass through, for instance by absorption or by reflection or refraction at the interfaces between media.

The term transmission medium also refers to a technical device that employs the material substance to transmit or guide waves. Thus, an optical fiber or a copper cable is a transmission medium. Not only this but also is able to guide the transmission of networks.

A transmission medium can be classified as a:

Linear medium, if different waves at any particular point in the medium can be superposed;

Bounded medium, if it is finite in extent, otherwise unbounded medium;

Uniform medium , if its physical properties are unchanged at different points;

Isotropic medium, if its physical properties are the same in different directions.

Coaxial cable, one example of a transmission medium.

Electromagnetic radiation can be transmitted through an optical media, such as optical fiber, or through twisted pair wires, coaxial cable, or dielectric-slab wave guides. It may also pass through any physical material that is transparent to the specific wavelength, such as water, air, glass, or concrete. Sound is, by definition, the vibration of matter, so it requires a physical medium for transmission, as does other kinds of mechanical waves and heat energy. Historically, science incorporated various aether theories to explain the transmission medium. However, it is now known that electromagnetic waves do not require a physical transmission medium, and so can travel through the "vacuum" of free space. Regions of the insulative vacuum can become conductive for electrical conduction through the presence of free electrons, holes, or ions.

TRANSMISSION MEDIA

The means through which data is transformed from one place to another is called transmission or communication media. There are two categories of transmission media used in computer communications.

BOUNDED/GUIDED MEDIA UNBOUNDED/UNGUIDED MEDIA

1. BOUNDED MEDIA:

Bounded media are the physical links through which signals are confined to narrow path. These are also called guide media. Bounded media are made up o a external conductor (Usually Copper) bounded by jacket material. Bounded media are great for LABS because they offer high speed, good security and low cast. However, some time they cannot be used due distance communication. Three common types of bounded media are used of the data transmission. These are

Coaxial Cable Twisted Pairs Cable Fiber Optics Cable

1.COAXIAL CABLE:

Coaxial cable is very common & widely used commutation media. For example TV wire is usually coaxial.

Coaxial cable gets its name because it contains two conductors that are parallel to each other. The center conductor in the cable is usually copper. The copper can be either a solid wire or stranded martial.

Outside this central Conductor is a non-conductive material. It is usually white, plastic material used to separate the inner Conductor form the outer Conductor. The other Conductor is a fine mesh made from Copper. It is used to help shield the cable form EMI.

Outside the copper mesh is the final protective cover. (as shown in Fig)

The actual data travels through the center conductor in the cable. EMI interference is caught by outer copper mesh. There are different types of coaxial cable vary by gauge & impedance.

Gauge is the measure of the cable thickness. It is measured by the Radio grade measurement, or RG number. The high the RG number, the thinner the central conductor core, the lower the number the thicker the core.

Here the most common coaxial standards.

50-Ohm RG-7 or RG-11 : used with thick Ethernet. 50-Ohm RG-58 : used with thin Ethernet 75-Ohm RG-59 : used with cable television 93-Ohm RG-62 : used with ARCNET.

CHARACTERISTICS OF COAXIAL CABLE

Low cost Easy to install Up to 10Mbps capacity Medium immunity form EMI Medium of attenuation

ADVANTAGES COAXIAL CABLE

Inexpensive Easy to wire Easy to expand Moderate level of EMI immunity

DISADVANTAGE COAXIAL CABLE

Single cable failure can take down an entire network

STP

2. Twisted Pair Cable

The most popular network cabling is Twisted pair. It is light weight, easy to install, inexpensive and support many different types of network. It also supports the speed of 100 mps. Twisted pair cabling is made of pairs of solid or stranded copper twisted along each other. The twists are done to reduce vulnerably to EMI and cross talk. The number of pairs in the cable depends on the type. The copper core is usually 22-AWG or 24-AWG, as measured on the American wire gauge standard. There are two types of twisted pairs cabling

Unshielded twisted pair (UTP) Shielded twisted pair (STP)

1. Unshielded twisted pair (UTP)

UTP is more common. It can be either voice grade or data grade depending on the condition. UTP cable normally has an impedance of 100 ohm. UTP cost less than STP and easily available due to its many use. There are five levels of data cabling

Category 1

These are used in telephone lines and low speed data cable.

Category 2

These cables can support up to 4 mps implementation.

Category 3

These cable supports up to 16 mps and are mostly used in 10 mps.

Category 4

These are used for large distance and high speed. It can support 20mps.

Category 5

This is the highest rating for UTP cable and can support up to 100mps

UTP cables consist of 2 or 4 pairs of twisted cable. Cable with 2 pair use RJ-11 connector and 4 pair cable use RJ-45 connector.

Characteristics of UTP

low cost easy to install High speed capacity High attenuation Effective to EMI 100 meter limit

Advantages of UTP

Easy installation Capable of high speed for LAN Low cost

Disadvantages of UTP

Short distance due to attenuation

2. Shielded twisted pair (STP)

It is similar to UTP but has a mesh shielding that’s protects it from EMI which allows for higher transmission rate.

IBM has defined category for STP cable.

Type 1

STP features two pairs of 22-AWG

Type 2

This type include type 1 with 4 telephone pairs

Type 3

This type feature two pairs of standard shielded 26-AWG

Type 4

This type of STP consist of 1 pair of standard shielded 26-AWG

Type 5

This type consist of shielded 26-AWG wire

Characteristics of STP

Medium cost Easy to install Higher capacity than UTP Higher attenuation, but same as UTP Medium immunity from EMI 100 meter limit

Advantages of STP:

Shielded Faster than UTP and coaxial

Disadvantages of STP:

More expensive than UTP and coaxial More difficult installation High attenuation rate

3.Fiber Optics

Fiber optic cable uses electrical signals to transmit data. It uses light. In fiber optic cable light only moves in one direction for two way communication to take place a second connection must be made between the two devices. It is actually two stands of cable. Each stand is responsible for one direction of communication. A laser at one device sends pulse of light through this cable to other device. These pulses translated into “1’s” and “0’s” at the other end.

In the center of fiber cable is a glass stand or core. The light from the laser moves through this glass to the other device around the internal core is a reflective material known as CLADDING. No light escapes the glass core because of this reflective cladding.

Fiber optic cable has bandwidth more than 2 gbps (Gigabytes per Second)

Characteristics Of Fiber Optic Cable:

Expensive Very hard to install Capable of extremely high speed Extremely low attenuation No EMI interference

Advantages Of Fiber Optic Cable:

Fast Low attenuation No EMI interference

Disadvantages Fiber Optics:

Very costly Hard to install

UNBOUNDED / UNGUIDED MEDIA

Unbounded / Unguided media or wireless media doesn't use any physical connectors between the two devices communicating. Usually the transmission is send through the atmosphere but sometime it can be just across the rule. Wireless media is used when a physical obstruction or distance blocks are used with normal cable media. The three types of wireless media are:

RADIO WAVES MICRO WAVES INFRARED WAVES

1. RADIO WAVES:-

It has frequency between 10 K Hz to 1 G Hz. Radio waves has the following types.

Short waves VHF (Very High Frequency) UHF (Ultra High Frequency)

1.SHORT WAVES:-

There are different types of antennas used for radio waves. Radio waves transmission can be divided into following categories.

LOW POWER, SINGLE FREQUENCY. HIGH POWER, SINGLE FREQUENCY

1. LOW POWER , SINGLE FREQUENCY:-

As the name shows this system transmits from one frequency and has low power out. The normal operating ranges on these devices are 20 to 25 meter.

CHARACTERISTICS LOW POWER , SINGLE FREQUENCY:-

Low cost Simple installation with pre-configured 1 M bps to 10 M bps capacity High attenuation Low immunity to EMI

2. HIGH POWER, SINGLE FREQUENCY:-

This is similar to low power single frequency. These devices can communicate over greater distances.

CHARACTERISTICS  HIGH POWER, SINGLE FREQUENCY:-

Moderate cost Easier to install than low power single frequency 1 Mbps to 10 Mbps of capacity Low attenuation for long distances Low immunity to EMI

2.MICRO WAVES

Micro waves travels at high frequency than radio waves and provide through put as a wireless network media. Micro wave transmission requires the sender to be inside of the receiver.

Following are the types of Micro waves.

1.· Terrestrial Micro waves2.· Satellite Micro waves

1.Terrestrial Micro waves:-

Terrestrial Micro waves are used are used to transmit wireless signals across a few miles. Terrestrial system requires that direct parabolic antennas can be pointed to each other. These systems operate in a low Giga Hertz range.

Characteristics of Terrestrial Micro waves:-

Moderate to high cost. Moderately difficult installation 1 M bps to 10 M bps capacity Variable attenuation Low immunity to EMI

2.Satellite Micro waves

The main problem with aero wave communication is the curvature of the earth, mountains & other structure often block the line of side. Due to this reason, many repeats are required for long distance which increases the cost of data transmission between the two points. This problem is recommended by using satellites.

Satellite micro wave transmission is used to transmit signals through out the world. These system use satellites in orbit about 50,000 Km above the earth. Satellite dishes are used to send the signals to the satellite where it is again send back down to the receiver satellite. These transmissions also use directional parabolic antenna’ with in line of side.

In satellite communication micro wave signals at 6 GHz is transmitted from a transmitter on the earth through the satellite position in space. By the time signal reaches the satellites becomes weaker due to 50,000 Km distance. The satellite amplifies week signals and transmits it back to the earth at the frequency less than 6 GHz.

Characteristics Satellite Micro waves: High cost Extremely difficult and hare installation. Variable attenuation. Low immunity to EMI High security needed because a signal send to satellite is broadcasts through all

receivers with in satellite.

3. Infrared

Infrared frequencies are just below visible light. These high frequencies allow high sped data transmission. This technology is similar to the use of a remote control for a TV. Infrared transmission can be affected by objects obstructing sender or receiver. These transmissions fall into two categories.

1. Point to point2. Broadcast

(i) Point to Point: - Point to point infrared transmission signal directly between two systems. Many lap top system use point to pint transmission. These systems require direct alignment between many devices.

Characteristics of Point to point:- Wide range of cost Moderately easy installation. 100 k bps to 16 Mb of capacity. Variable attenuation. High immunity to EMI

(ii) Broad Cast: - These infrared transmission use sprayed signal, one broad cast in all directions instead of direct beam. This help to reduce the problems of proper alignment and abstraction. It also allows multiple receiver of signal

Characteristics of Broad Cast:-

In expensive. Single installation. 1M bps capacity. Variable attenuation.